Karl Fischer Titrators

In volumetric Karl Fischer Titration, iodine is added by a moisturized piston burette during titration. It is suitable for samples where water is present as a major component and measures ranges from 100ppm-100%.

Coulometric Karl Fischer Analysis generates iodine via electrochemical oxidation in the cell during titration. It is suitable for samples where the water is present in trace amounts, measuring ranges from 1ppm-5%.

With the one click start of your Karl Fischer analysis, the pre titration is executed automatically by the method. During this step, the sample determination can not be started and the keys on the terminal will appear grey. Once the pre titration is finished and no water is present in the cell anymore, the titrator switches in the standby mode and the keys will appear white to signal that the sample can now be titrated.

The C20 and C30 are available with two different coulometer cells – with or without a diaphragm. For most applications, we recommend the cell without the diaphragm because it is almost maintenance-free. Due to its innovative design, this diaphragm-free cell from METTLER TOLEDO can even be used for the determination of water in oils.

The version of the cell with a diaphragm is recommended for applications such as the determination of moisture in substances containing ketones. It is also recommended if the best possible accuracy is required.

Naturally, this depends on the stability of the titrant and on what measures have been taken to protect the titrant from the typical contaminants that could cause a reduction in concentration. The most common examples of this titrant protection are the storage of light sensitive titrants in dark bottles e.g. iodine solutions, the protection of Karl Fischer titrants from moisture using e.g. molecular sieve or silica gel, and the protection of certain strong bases e.g. sodium hydroxide, from absorption of carbon dioxide.

The most practical solution to this question is to add some blue silica gel to the top of the drying tube to serve as an indicator. As soon as the first trace of pink appears in this gel layer, it is time to change or regenerate the molecular sieve. Naturally, an increase in background drift can also indicate that it is time to replace the molecular sieve.

When validating a water titration method one needs to check things like accuracy, precision, reproducibility, linearity, systematic errors, robustness, ruggedness and limits of determination. For detailed recommendations on how to go about this validation please refer to our section on Quality Control, Validation or refer to the METTLER TOLEDO applications brochure 16 - Validation of Titration methods.

For standardizing Karl Fischer titration reagents, one would expect the best to be pure water standard. Water, however, does not satisfy the requirements of a primary standard in that it is not stable during weighing and does not have a high enough molecular weight. The second point raises the problem of accurately weighing a sufficiently small sample so as to give a reasonable titrant consumption.

As an alternative to pure water, certified standard solutions can be obtained in various concentrations ranging from 0.1mg to 10mg water per g (or per mL). This allows the weighing out of a more suitable sample size.

A third possibility is to use a solid sample with exactly known water content. The most common standard here is sodium tartrate dihydrate. This standard contains two waters of crystallization giving it a water content of exactly 15.66%. The advantage of this standard is that it is available as a finely ground powder with a stable and guaranteed water content. Since the moisture content is only 15.66% as opposed to the 100% pure water, one is able to weigh a reasonable sample size with sufficient accuracy to ensure a good titer determination. The only drawback of this standard is that it has limited solubility in methanol, which is the most common Karl Fischer solvent used. As a rule, approximately 0.15g of standard will dissolve in 40mL of methanol. Increasing results for the concentration determination on successive samples indicates incomplete dissolution. Provided this limited solubility is taken into account, the sodium tartrate dihydrate is the standard of choice for Karl Fischer titration reagent concentration determination.

In Karl Fischer Titration, the best method to weight your sample is by back -weighting. As a general rule, one should have at least 4 significant figures for the sample weight.

The first and most obvious answer to this question is that the solvent should be replaced as soon as the sample no longer dissolves. This, however, is only one of the reasons for changing the solvent. A second less obvious reason applies in the case of two component reagent where the titrant contains iodine, and the solvent contains all the other components necessary for the Karl Fischer reaction. One of these other components is sulfur dioxide and this can become depleted long before the dissolving capacity of the solvent is exceeded. As a general rule the solvent in these two component systems has an approximate water capacity of 7mg of water per mL of solvent. This means that in theory 40mL of solvent can accommodate 280mg of water before the solvent need be changed. As the typical titrant has a concentration of 5mg/mL, 280mg of water would require 56mL of titrant.

Karl Fischer titration can be used in a variety of industries, one of them being the pharmaceuticals industry in which water content is not only important in understanding the performance of drug substances and drug products but is also a limiting factor to the accuracy of drug content calculated on an anhydrous and solvent-free basis. For example, as cannabis and its natural substance CBD nowadays gains more popularity on the market, KF titration is an important tool for the quality of the CBD oil or extracts as it can also be applied in the industry of cosmetics. Further is Karl Fisher titration used in the food industry, given that the water content affects the physical and chemical aspects of food, which is directly related to freshness and stability for the storage of the food over a long period of time. To some degree, water is also always present in fuel system or bulk storage tanks which is why Karl Fischer titration can also be also used in the industry of Petroleum.

The Karl Fischer Titration is based on the reaction described by the Bunsen reaction, described by R. W. Bunsen:

I₂ + SO₂ + 2 H₂O → 2 HI + H₂SO₄

Karl Fischer discovered that this reaction could be used for water determinations in a non-aqueous system containing an excess of sulfur dioxide. In order to achieve an equilibrium shift to the right, it is necessary to neutralize the acids that are formed during the process. Karl Fischer used pyridine for this purpose, however, pyridine is not directly involved into the reaction and acts merely as buffering agent, which is why it can be replaced by other bases. Studies by E. Scholz resulted in the replacement of the toxic, pungent pyridine by imidazole, which proved to lead to faster and more accurate titrations as imidazole buffers in a more favorable pH range than pyridine.

Methanol proved to be suitable as a solvent giving that it not only acts as such, but also participates directly in the reaction itself. In an alcoholic solution, the reaction between iodine and water takes place in the stochiometric ratio of 1:1. In an alcohol-free solution, the reaction between iodine and water takes place in the stochiometric ration of 1:2.

This resulted in the general chemical equation:

ROH + SO₂+ 3 RN + I₂+ H₂O → (RNH)•SO₄ R + 2 (RNH)I

Yes. The KF titration can be influenced by 3 main parameters: the pH, the solvent as well as the influence of water content of the sample as it also influences the H₂O : I₂ molar ratio.

No, the InMotion KF Oven can only be used with the Compact V30S as well as the T7 and T9 Excellence Titrators.

Air humidity is the big enemy of Karl Fischer Titration. Make sure all the openings on the Titrator have been greased (e.g. tube inserts, electrodes, adapters…) and that you use dry and fresh molecular sieves and silica gel in your drying tubes. We recommend exchanging your molecular sieves every 6 weeks. If you still have high values, try to gently shake the titration vessel to collect traces of water from the glass walls.

Easy: The Compact C10/20/30SX Karl Fischer Titrators do not possess a diaphragm. Without it, there’s no risk on getting it clogged and hence receive incorrect results. The C10/20/30SD Compact Titrators possess a diaphragm, which can improve the drift value.